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  • Single-Cell Boost Converter Circuit

  • Created: Nov 04, 2014

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Summary

Single cell boost converter is a device that provides a higher voltage output from a single cell battery. The device uses a MAX856 CMOS , step-up DC-DC switching regulator for small, low input voltage. It has a quiescent current of 22uA with efficiency of 85% at 100mA. Its ultra low quiescent supply current and high efficiency gives maxim battery life. In addition, the regulator employs low-battery detector (a comparator/reference combination) as an on/off controller for transistor Q1. Normally, the input (LBI) monitors a positive battery voltage and drives the output (LBO) low when LBI drops below 1.25V.

The main 3.3V supply is provided by a boost converter (U1) operating in its standard configuration. The auxiliary bias voltage is provided by an extra flyback winding and is regulated via Q1 and the low-battery detector internal to U1. As the battery discharges, its declining terminal voltage causes a decline in the voltage induced in the flyback winding. At minimum battery voltage (0.8V), the T1 primary sees 3.3V - 0.8V = 2.5V; thus, the 6:1 turns ratio produces 6 × 2.5 = 15V in the secondary. At maximum battery voltage (1.65V), the primary sees only 1.65V, producing 9.9V in the secondary. MOSFET Q1 stabilizes this output by interrupting the secondary current, introducing the regulation necessary to generate a constant negative output. Moreover, the R1/R2 divider holds LBI between VCTRL (normally 3.3V) and the LCD bias output (normally -8V). The R1 and R2 values are chosen such that LBO turns Q1 off when the LCD bias becomes too negative (and pulls the LBI voltage below 1.25V). Load current then causes the LCD bias to drift upward (toward 0V) until LBI exceeds 1.25V, which causes Q1 to turn on again. Because ripple is essential to operation in this hysteretic converter, it cannot be reduced directly.

The circuit generates two supply voltages commonly used to bias LCDs such as that in pagers and other portable instruments that have small, graphic liquid-crystal displays. The regulated 3.3V at 100mA, and a regulated negative output will be as an LCD bias voltage. The main voltage can be changed from 3.3V to 5V by grounding the 3/active-low 5 terminal on U1. However, the turns ratio should also be reduced to 3:1 because the highest battery voltage will induce 3.35V in the T1 primary. By adjusting the R1 and R2 values, the desired negative-output level will be obtained.





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